This invention relates to improvements in or relating to apparatus for use in assembling electrical components on printed circuit boards.
A known form of apparatus used in the assembly of electronic components on printed circuit boards comprises an inclined working frame which is conveniently arranged for a seated operative, and on which the printed circuit board is secured. The printed circuit board has pre-drilled holes at predetermined locations for receiving the leads of corresponding components. An optical projector is mounted above the working surface and is controlled by a micro-computer in accordance with a predetermined program to project light spots onto the printed circuit board to indicate to an operator, in sequence, the positions at which the various components are to be assembled on the board. Means may also be provided to indicate the identify of the component to be assembled at that location.
In use of such apparatus, the leads of the selected component are inserted by the operator into the holes indicated by the projector and the projector program is then stepped to the next position. This sequence of operations is repeated until all the required components have been assembled on the board.
It is then necessary to cut short the projecting leads on the undersurface of the board. This can be done by cropping and bending (clinching) each lead individually, thereby securing the assembled components to the board. Alternatively, all of the assembled components to be cut may be held in position on the board by means of a resilient pad applied to the upper surface of the board, while all the leads are cropped in one sweep by a single wide cutting blade. In the latter case, because the leads are not clinched, it is then necessary to use some other means (commonly a shrink wrap foil) to secure the assembled components while the board is soldered, commonly on a wave soldering machine.
In practice, the individual lead cropping and clinching approach is time-consuming compared to the time required to assemble the components on the board, while the single wide cutting blade approach requires some additional means of securing the components prior to soldering and presents significant manufacturing difficulties.
Because of the foregoing disadvantages, a variant of the optically assisted assembly apparatus has been developed in which the printed circuit board (B) is movable with the working frame in mutually perpendicular X and Y directions under program control as illustrated in FIG. 2 of the accompanying drawings. Two cutters (C) are mounted symmetrically under the printed circuit board on a carrier (K) which is vertically movable through a short (typically 5 mm) distance and which is also rotatable about a fixed vertical reference axis (R). In use, the circuit board is positioned so that the reference axis concides substantially with the centre of the component to be inserted, that is coincides with the mid-point between the corresponding printed circuit board holes (H). The distance (pitch p) between the cutters can then be adjusted and the carrier rotated (.theta.) about the reference axis under program control so that nips of the cutters are aligned with the printed circuit board holes. After alignment, the cutter assembly is raised to bring the cutters into contact with the underside of the printed circuit board.
As the component leads are inserted in the corresponding printed circuit board holes, the leads also enter the nips of the cutters positioned beneath the printed circuit board holes. On command, the cutters operate and crop the leads of the component to the required length and also clinch the cropped leads to secure the component to the board. This process is repeated in turn with each component. Thus, at the completion of the assembly operation, the board is immediately ready for flow-soldering without any possibility of components becoming detached from the board.
In the known apparatus provided with such programmed cutters, in order to accomodate very short component pitches, it is necessary to arrange the cutters so that they can pass each other at the centre of rotation defined by the reference axis. Because the minimum pitch required can be as small as 0.100 inches (2.54 mm), it can be seen that each cutter must protrude somewhat less than 0.050 inches (1.27 mm) beyond the centre-line of the component lead involved. As illustrated in FIG. 3 of the accompanying drawings, such an arrangement implies that the line (cutter axis) joining the centres of the nips of the two cutters (C) is not parallel with the line (hole axis) joining the centres of the two printed circuit board holes (H) and variable adjustment of the cutter assembly about the vertical reference axis (R) is required to compensate for this and bring the cutters into alignment with the printed circuit board holes. In effect, as shown on FIG. 3, the cutter axis is angularly offset from the hole axis by the variable angle ##EQU1## where the pitch is measured in mms.
With these constraints, a difficulty arises as regards the size of the cutters. Thus, the cutters must be small enough to fit between the leads of adjacent components which have already been assembled, so that the design of the printed circuit board is not constrained by the limitations of the assembly machine and, at the same time, the cutters must be strong enough (therefore large enough) to withstand the cutting loads encountered, which may be considerable when cutting maximum diameter copper leads or leads made of harder materials.
It is found that, in order to provide sufficiently strong cutters and to reduce the interference between cutters and component leads already assembled, it is necessary to minimise all the working clearances and this implies that the cutters must be accurately aligned with the corresponding holes by the programmed movements of the board and cutters in order to permit entry of the component leads.
The accuracy of the programmed alignment is a function, inter alia, of the accuracy of the following factors in the apparatus:
a) The X and Y position including the precision of the drive means and the straightness and perpendicularity of the guide means involved in positioning the circuit board. PA1 b) The distance between the cutters PA1 c) The angle of the line joining the cutters with respect to X and Y.
With these factors alone, the required accuracy will only allow errors in each element in the region of 0.025 mm, which is expensive to achieve.
In addition, there are sometimes significant errors in the drilling of the component holes in the printed circuit board. Such errors may not be consistent between nominally identical boards and cannot therefore be effectively compensated by adjustments to the programme.
Accumulation of these errors, possibly compounded by long term drift, sometimes makes it difficult for the operator to insert the leads through the cutters and can involve time consuming individual adjustment of the alignment of the cutters using manual joystick control.
In addition, the machine may require recalibration at intervals in order to maintain sufficient accuracy.